1. Field of the Invention
Present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus and method having a self test image printing function and a printing density adjustment function.
2. Related Art
In recent years, electrophotographic printers such as laser-beam printers (hereinafter, abbreviated to LBP) have been widely accepted in the market by virtue of their high-speed and high-resolution printing.
With regard to printing resolution, the resolution of a LBP widely used so far is 300 dpi (300 dots per inch). Recently, a LBP having a 600 dpi resolution has come out by request of many users for a higher resolution.
The problem is how to raise the relative precision of the size of one dot (dot diameter) among high-resolution LBP's. In a LBP, the dot diameter depends upon various parameters such as a laser-beam diameter and a laser-beam intensity (hereinafter referred to as a "laser quantity"), the sensitivity of a photoreceptor, the mechanical precision of a developing unit and the value of a developing high-voltage. The higher the resolution becomes, the more difficult it is to reduce variations of a dot diameter among LBP's.
In order to solve this problem, conventionally, a dot diameter regulating mechanism, i.e., a printing density adjustment mechanism has been employed. For example, LBP's are equipped with a rheostat to change the developing high voltage or the laser quantity by changing the resistance of the rheostat.
FIG. 1 is a block diagram showing a conventional image forming apparatus which controls the image density by a slide rheostat. In this apparatus, a voltage controlled by the slide rheostat is directly input into a high-voltage power supply circuit. The high-voltage power supply circuit controls various voltages to adjust the image density. As shown in FIG. 1, the image forming apparatus comprises a host I/F circuit which receives information from a host PC, a formatter which expands printing information from the host PC into image data, an engine, and a console panel with which an operator controls the image forming apparatus. The formatter controls the engine, which controls the electrophotographic process and prints the image information received from the formatter on a recording sheet. In the above structure, the slide rheostat is employed to change a primary charging voltage and the developing voltage of the high-voltage power supply circuit, and to change the toner density or laser quantity in developing the toner upon an electrostatic drum, controlling the printing density.
However, in the above conventional apparatus, the operator should repeat the printing operation by changing the value of the rheostat until the operator obtains a desired printing density.
Further, when old photoreceptor and developing unit are exchanged for new ones, the rheostat of the apparatus should be adjusted corresponding to the new units.
In the conventional apparatus, in order to control the image density, the voltage set by the slide rheostat is directly input into the high-voltage power supply, and the toner density and the latent image intensity are changed in accordance with the input voltage. If the slide rheostat is operated in mid-course of image formation, the image density is changed again, causing poor image condition. Accordingly, the slide rheostat is arranged in a position disabling the control during image formation, however, the user cannot easily operate the slide rheostat.
Further, in a system where a plurality of operators use a host PC, or in a system where a plurality of host PC's are connected to an image forming apparatus, it is necessary to check the value of the rheostat at each printing. If the value set in the previous printing is different from that for the current printing, the value must be set again.